This invention relates to an apparatus for mounting an integrated circuit (IC) to a printed circuit board. In particular, this invention is embodied in a demountable tape automated bonded (DTAB) IC mounting device.
Every year, IC technology advances and the ICs designed and manufactured become more complex. Today, designers are designing and manufacturing ICs incorporating over 300,000 transistors. To fully utilize the capabilities of these highly complex ICs, the IC packaging technology must also keep pace. Large complex ICs require a large number of input and output (I/O) connections. An IC with a size of 0.5 inches square requiring 400 or more I/O connections is not uncommon.
One technology for mass-producing connections to ICs is called "Tape-automated bonding" (TAB) and is well known to persons ordinarily skilled in the electronics packaging art. TAB technology uses a continuous insulative tape to provide a planar support for IC chips that are attached to individual sections or frames of the tape. TAB frames are generally rectangular or square sections that are arranged side-by-side along an uncut tape. A spider-like metal pattern of conductive traces is formed on each frame. The traces radiate from the center of the frame to its four edges. An IC chip is attached to the center of the TAB frame so the leads or contacts of the chip are precisely mated with the corresponding metal traces in the central portion of the TAB frame. A TAB frame is mounted on a printed circuit board (PCB) thereby mating the metal traces of the TAB with metal traces on the PCB. The resulting assembly comprising the chip, the TAB frame and the PCB, is essentially a space transformer that employs diverging radial electrical pathways to afford ready access to the IC chip.
Conventional TAB technology is based on soldering the tape outer leads to the matching pattern on the PCB. This soldering operation is difficult to accomplish, and makes rework or repair of the PCB virtually impossible in production conditions. Therefore, there is a demand in the industry for an TAB attachment method that is easily reworkable and readily demountable.
A prior art demountable TAB system is disclosed in U.S. Pat. No. 4,658,331 to Berg. FIG. 1 shows the structure disclosed in the Berg patent. As shown in FIG. 1, an IC chip 102 is mounted on a TAB frame 114 and the TAB frame 114 is clamped onto a surface of the PCB 104 by elastomer pressure pads 112 and screws 110. The pressure pads 112 serve to bias the contacts (not shown) on the TAB frame 114 against the contacts (not shown) of the PCB 104. Extensions 116 of the TAB frame 114 are used to visually align the TAB frame 114 to the contacts (not shown) of the PCB 104. A heat spreader 140 is attached to the IC chip 2 to aid in heat dissipation.
The prior art demountable TAB system has significant drawbacks. To produce the necessary contact force between the contacts on the TAB frame 114 and the PCB 104, the elastomeric pressure pads 112 have to be compressed significantly. The uncompressed height of the elastomer is so great, that when the chip 102 is attached to the heat spreader 140, the TAB must take the form of an inverted truncated pyramid. As the four corners of the TAB frame are typically held fixed to the PCB by mounting hardware, raising the center of the TAB frame to form a pyramid shape subjects the TAB frame to high stresses and curling and causes the loss of alignment to the underlying PCB 104.
To reduce the stress forces a TAB frame is subjected to, the elastomer spring height could be reduced. However, a significant reduction in the uncompressed height of the elastomer is not possible since the spring force becomes too small to be controllable. If the amount of compression is small, for example 10 mils, then a small variation of 1-2 mils, resulting from the bending of the heat spreader under load, causes a 10-20% loss of contact force. Such an assembly is likely to fail during thermal cycling caused by the IC chip heating up and cooling off during use.
The prior art demountable TAB system does not lend itself well to repeated mounting operations. After the mounting system has been in use and subjected to thermal aging or cycling, the elastomer tends to take a set. The elastomer will no longer return to the original height once the clamping force is removed. As such, the bias force supplied by the elastomer will decrease. Also, the elastomer will become imprinted with the TAB contact pattern and will not supply an even bias force to the TAB contacts if reused.
Another drawback of the prior art demountable TAB is the use of tape extensions 116 to align the TAB frame 114 with the PCB 104. This method requires a larger and more expensive TAB frame than is required for the TAB frame contacts alone and requires sophisticated optical alignment equipment.
A drawback of all the prior art TAB systems is the mandated use of fine line PCBs for high lead count applications. Fine line PCB technology refers to the use of PCB traces that are spaced apart by less than about 0.02 inches. If the traces are spaced less than 0.02 inches, the PCB manufacturing processes must be tightly controlled. The tighter the process must be controlled, the more costly are the PCBs.
FIG. 2 shows one of four sides of a prior art TAB peripheral edge 202 and a mating PCB contact pattern 204. Each PCB connection 208 is spaced apart from the adjacent connection 208 by approximately 0.01 inches and is typically connected to a through hole 206. The through holes 206 are spaced apart by approximately 0.1 inches and are arranged in a grid 210. This grid 210 of through holes is required to permit software currently used in the industry to route the PCB connections to various locations on the PCB. No matter how small the TAB assembly is physically, the effective PCB area occupied by the TAB assembly is defined by the through hole grid 210. So, while the PCB area needed for the TAB hardware is defined by the through holes spaced 0.1 inches apart, the TAB frame must be precisely aligned to the PCB contacts that are spaced 0.01 inches apart, a difficult task.
With the above in mind, a need exists in the industry for a reusable TAB mounting assembly that is reliable, can withstand repeated thermal cycling and eliminates the need for fine line PCBs and the tedious optical alignment of the TAB frame to the PCB.